Observe the following tables, what is the foreign key between supplier and shipment table?

Supplier Table

SNUM

SNAME

STATUS

CITY

S$1$

Smith

$20$

London

S$2$

Jones

$10$

Paris

S$3$

Blake

$30$

Paris

S$4$

Clark

$20$

London

S$5$

Adams

$30$

Athens

Shipment Table

SNUM

PNUM

QTY

S$1$

P$1$

$300$

S$1$

P$2$

$200$

S$1$

P$6$

$100$

S$2$

P$1$

$300$

S$2$

P$2$

$400$

S$3$

P$2$

$200$

S$4$

P$2$

$200$

S$4$

P$4$

$300$

Part Table

PNUM

PNAME

COLOR

WEIGHT

P$1$

Nut

Red

$12$

P$2$

Bolt

Green

$17$

P$3$

Screw

Blue

$17$

P$4$

Screw

Red

$14$

P$5$

Cam

Blue

$12$

P$6$

Cog

Red

$19$